Because of the size and weight of semi-trucks, downhill braking can be difficult. A fully loaded 18-wheeler can weigh 80,000 pounds or more. As it descends a hill, the weight can make it gain speed quickly and take longer to brake and slow down.
A passenger car that’s traveling 65 mph needs about 300 feet to stop. But a fully loaded big rig truck going the same speed takes twice as long to stop, about 600 feet. That’s two football fields!